YAPB

I’m using Firefox as web browser, Thunderbird as email client and Pidgin as IM client.
Useful Add-Ons for Firefox:

• Download Statusbar (Moves the annoying Download window into a status bar)
• DownThemAll! (A pretty cool download accelerator directly in FireFox)
• FasterFox (Accelerates page load on broadband connections)
• FirePhish Anti-Phishing Extension (Anti-Phishing)
• ForecastFox Enhanced (See the weather in you FireFox)
• Google Browser Sync (Synchronize all you data with Google)
• IE Tab (If you have problems viewing pages with Gecko you can switch to IE rendering)
• Adblock Plus (Blocks most of the crap out there)
• Adblock Filterset.G Updater (Updater for Adblock)

Useful Add-Ons for Thunderbird:

• Buttons! (Add some cool buttons. I use it for Outlook style Next and Previous message buttons.)
• Colored Diffs (Will give you a formatted view of diff mails as generated by most SVNs)
• Dictionary Switcher (Switch dictionaries on the fly)
• Display Quota (Displays the IMAP quota)
• Lightning (The build-in calendar for Thunderbird)
• Minimize To Tray Enhancer (Enhances the tray options)
• MinimizeToTray (Minimize Thunderbird to tray)
• Provider for Google Calendar (Provider for Google calendars. Both way sync!)
• Quote Colors (Displays the mail quotes in a better way)
• XNote (Add notes to your mails. Not the best solution but it’s the best I could find)
• SyncKolab (Synchronize you contacts with multiple clients)

Pidgin plugins:

• Guifications Plugin Pack (Contains a lot of plugins in it)